Well, for me on Firefox 50.0b6, Chrome 54 and IE 11 on Win 8.1 the previews look the same, but then there are some differences when you check a specific one - for example the smiley has an upper lip on Firefox - http://cssicon.space/#/icon/smiley :) http://i.imgur.com/XlqKPhW.png.

Still the idea is very cool, you can basicly do anything with CSS (if anyone is interested check https://cssquests.com/ , the name is self-explanatory :)

From a performance perspective, how do those compare to using SVG icons?